1. Field of the Invention
The present invention relates to a structure of spline-connection for use in a power-transmission device, and specifically to a structure of spline-connection which is suitably used for a power transmission device for example a multiple disc clutch, consisting of an alternating series of clutch plates and discs, in which one of the engaging pair and another rotational-force transmitting member are both splined to a clutch drum with a plurality of splines (longitudinal multiple keyways or spline grooves) machined on its inner periphery.
2. Description of the Prior Art
In recent years, on power-transmission devices, a plurality of power-transmitting elements are combined with each other by way of spline connection for fulfillment of more complicated power flow regardless of a simple structure of the power train. This type of power transmission device has been disclosed in Japanese Patent Provisional Publication No. 60-84444. FIG. 4 shows the same power-transmission device as indicated in the Japanese Patent Provisional Publication No. 60-84444. In the power-transmission device described in terms of an automatic speed change gear, as shown in FIG. 4, a certain rotating body (a rotary ring gear 6R), a rotary race 18, and a rotary clutch drum 16 are mechanically coupled with each other. The clutch drum 16 is integrally formed on its inner periphery with spline grooves into which mated disc-clutch plates are fitted. The ring gear 6R has several projections (external spline keys) cut on its outer periphery to axially slidably support thereon mating disc-clutch plates. Also, a planet pinion carrier 4C of a first planetary gears 4 is coupled through the rotary inner race 18 with the clutch drum 16. The inner race 18 has several external splines being mated with the spline grooves of the clutch drum 16. Hereinbelow described in more detail is the structure of the automatic speed change gear of the prior art, by reference to FIG. 4. As seen in FIG. 4, the first and second planetary gearsets 4 and 6 are arranged concentric to the input shaft 2. The first planetary gearset 4, is comprised of a central sun gear 4S, the planet pinion carrier 4C, a plurality of pinion gears (not numbered) and the ring gear 4R (the internal gear). A band brake (not shown) is usually provided to permit or stop rotation of the sun gear 4S. The planet pinion carrier 4C is properly engaged with the input shaft 2 by way of a clutch device (not shown) and is held stationary by engaging with the transmission case 10 by means of a one-way clutch 8. The ring gear 4R is fixedly connected to a planet pinion carrier 6C of the second planetary gearset 6. The sun gear 6S of the second planetary gearset 6 is splined to the transmission input shaft 2, while the planet pinion carrier 6C of the second planetary gearset 6 is splined to the transmission output shaft 12. The ring gear 6R is designed to be connectable to or disconnectable from the sun gear 4S of the first planetary gearset 4 through the multiple disc clutch pack 14 placed on the outer periphery of the ring gear 6R, the clutch drum 16 placed on the outer periphery of the disc clutch pack 14, and the rotary inner race 18 of the one-way clutch 8. As seen in FIG. 4, several internal splines are formed integral with the clutch drum 16 and usually consist of circumferentially equally-spaced keyways cut on the inner periphery of the drum 16. Similarly, several external splines are formed integral with the ring gear 6R and usually consist of circumferentially equally-spaced keys cut on the outer periphery of the ring gear 6R. The multiple disc clutch pack 14 is comprised of an alternating series of clutch plates (driven plates) and discs (driving discs), and a pressure plate. Each of the driven plates is keyed on its outer edge, whereas each of the driving discs is keyed on its inner edge. The keyed sections of the driven plates mounted on the inner periphery of the clutch drum are often called "external splines or spline keys". The keyed sections of the driving discs mounted on the outer periphery of the ring gear are often called "internal splines or spline keyways". The external splines of the driven plates of the clutch drum side are fitted into the internal splines of the clutch drum 16, while the internal splines of the driving discs of the ring gear side are fitted onto the external splines of the rotary ring gear 6R. Such spline connection permits axial sliding movement of each of the clutch plates and discs in the axial directions of the axis P of the transmission input or output shafts. On the other hand, the rotary inner race 18 is integrally formed with several radially outwardly projecting external splines 18a being splined to the internal splines of the clutch drum 16 to permit the axial sliding movement of the clutch drum 16 relative to the inner race 18. When working fluid (hydraulic oil) is supplied into a pressure chamber 20 defined in the clutch drum and thus a piston 22 with inner and outer piston seal rings moves rightwards (viewing FIG. 4) against the bias of a piston spring or springs 22a with oil pressure applied to the piston 22, the driven plates and driving discs are engaged with each other. This allows power or torque to flow from the transmission input shaft through the sun gear 4S of the first planetary gearset 4, the inner race 18, the clutch drum 16 and the multiple disc clutch 14 to the ring gear 6R of the second planetary gearset 6. Conversely, when the working fluid flows from the cylinder chamber 20 back to the return line and thus the pressure in the chamber 20 is reduced below a predetermined pressure level, the clutch is held by virtue of the bias of the piston spring 22a in its disengaged position in which the driving discs rotate freely between the driven plates.
In the prior art power-transmission device as discussed previously, the external splines 18a of the inner race 18 are additionally splined to the internal splines of the clutch drum 16. The axial length T.sub.1 (the external spline width) of each of the splines of the inner race 18 is dimensioned to be a comparatively greater value in such a manner as to increase a spline contact area between the mating pair namely a certain external spline of the inner race 18 and the associated internal spline of the clutch drum 16. The increased axial length T.sub.1 of the splines 18a causes the pressure between the surfaces in contact, that is, the side wall surface of the external spline 18a and the side wall surface of the internal spline of the clutch drum 16, during power-transmission operation, to be lowered effectively, thus increasing the life and mechanical strength of the splines 18a. However, the increased axial length T.sub.1 of the splines 18a simultaneously results in the increase in the total thickness of the inner race itself. This prevents the entire size and dimensions of the power-transmission device from being small-sized or lightened. Additionally, in case of the prior art power-transmission device shown in FIG. 4, the clutch drum 16 is equipped at the right end with a snap ring 24 so that the left-hand side wall of the snap ring 24 faces to the right-hand face of each of the external splines 18a of the inner race. The snap ring 24 functions to restrict the maximum axial leftward movement of the drum 16 but not the rightward movement. Therefore, there is a possibility that the drum 16 repeatedly moves rapidly momentarily between an axially spaced position, in which the drum 16 is shifted axially rightwards and thus there is a clearance between the snap ring 24 and the right-hand side wall of the respective external spline 18a, to an abutment position, in which the drum 16 moves axially leftwards together with the piston 22 by way of the bias of the piston spring 22a and thus the snap ring 24 abuts the external splines 18a. This movement may causes undesired chattering noise, and thus the driver may feel uncomfortable.